This invention relates to the separation of isotopes, and more particularly, it relates to isotope separation methods and apparatus wherein optical excitation is used to select the desired isotope.
The separation, or at least the enrichment, of particular isotopic forms of certain chemical elements has been of interest to the scientific community for several decades. However, in recent years, on account of the importance of obtaining enrichment of the fissionable uranium isotope U.sup.235 for nuclear applications, a vast research and development effort has been under way to advance the state of the art of isotope separation in general and the separation of uranium isotopes U.sup.235 and U.sup.238 in particular.
In the past many schemes for isotope separation have been proposed and demonstrated including separation by gaseous diffusion through a porous barrier, electromagnetic separation using a mass spectrometer, centrifugal separation, separation by thermal diffusion, separation by fractional distillation, electrolytic separation, and chemical separation using isotopic exchange reactions with other elements.
A practical problem encountered with many of the aforementioned isotope separation techniques, e.g., the gaseous diffusion method, is that the enrichment ratio of the isotopes being processed (i.e., the ratio of the percentage of the desired isotope in the output mixture to the percentage of the desired isotope in the input mixture) is quite low. As a result, an extremely large number of stages are needed to obtain useful levels of enrichment of the desired isotope. For example, a ten-fold enrichment of the uranium isotope U.sup.235 from its naturally occurring level of 0.72% would require about 1800 gaseous diffusion stages. Other of the aforementioned isotopic separation techniques, e.g., electromagnetic separation, provide substantially higher enrichment ratios than the gaseous diffusion process. However, such high enrichment ratio processes are capable of handling only small amounts of isotopic material and, therefore, are impractical for high volume use.
Recently, several isotope separation schemes have been devised based on selective optical excitation of a desired isotope in a mixture of isotopes using a tunable laser. In these schemes the laser is tuned so that its output coincides in frequency with an allowed transition of the desired isotopic species but not with that of the undesired isotopic species. The selectively excited isotopic species is subsequently ionized by either absorption of light (supplied from a second laser or an incoherent source) or by contact with a heated ionizing surface. Ions are thus produced from the excited isotope only, and these ions are physically separated from the mixture by deflection using either an electric or magnetic field, or both. In order for the deflecting forces from the electric or magnetic fields to be efficiently applied, the original isotopic mixture must exist in the form of an atomic or molecular beam so as to minimize charge exchange collisions which might scramble the excitation (i.e., ionize undesirable isotopes and deionize desired isotopes). A trade-off exists such that high isotope enrichment ratios can be achieved only at the expense of processing a very small amount of material. Further details concerning the aforementioned selective optical excitation schemes may be found in Robieux et al U.S. Pat. No. 3,443,087, Levy et al U.S. Pat. No. 3,772,519 and Pressman U.S. Pat. No. 3,740,552.